1. Field of the Invention
This invention relates to gas turbine engines, and more particularly, to a diffusion barrier layer applied to airfoils in the turbine portion of a gas turbine engine.
2. Discussion of the Prior Art
The current coatings used on airfoils exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in thermal barrier coating (TBC) systems include both diffusion aluminides and MCrAlY (X) coatings. These coatings are applied over substrate materials, typically nickel-based superalloys, to provide protection against oxidation and corrosion attack. These coatings are formed on the substrate in a number of different ways. For example, a nickel aluminide, NiAl, may be grown as an outer coat on a nickel based superalloy by simply exposing the substrate to an aluminum rich environment at elevated temperatures. The aluminum diffuses into the substrate and combines with the nickel to form a coating of NiAl on the outer surface. A platinum-containing nickel aluminide (Pt--NiAl) coating can be formed by electroplating platinum over the nickel-base substrate to a predetermined thickness. Exposure of the platinum-coated substrate to an aluminum-rich environment at elevated temperatures causes the growth of an outer layer of Pt--NiAl as the aluminum diffuses into and reacts with the platinum and the underlying substrate. At the same time, Ni diffuses outward from the substrate. Depending on the Al content of the gas phase, as the aluminum diffuses toward the substrate and the Ni diffuses away from the substrate and into the coating, PtAl.sub.x phases may precipitate so that the Pt--NiAl intermetallic may also contain precipitates of PtAl.sub.x intermetallic, where x is 2 or 3.
Of course, an MCrAlY where M is an element selected from the group consisting of Ni, Fe and Co and combinations thereof may be applied to the substrate as a bond coat or as an environmental coating by any known technique. When applied as bond coats in thermal barrier systems, an additional thermally resistant ceramic coating such as yttria-stabilized zirconia (YSZ) is applied over top of the coating. As the airfoils are exposed to the hot, oxidative, corrosive environment of a gas turbine engine, a number of metallurgical processes modify the airfoils. Initially, the aluminum rich bond coat forms a highly adherent alumina (Al.sub.2 O.sub.3) layer which grows under the ceramic coating. However, with further high temperature engine service, spallation of the YSZ topcoat occurs at either the bond coat/alumina interface or at the alumina/YSZ interface. The strength of these interfaces, the stresses in the interface plane, and their changes with temperature exposure can influence the TBC coating system life.
There are many factors related to chemistry and microstructure of both the alloy substrate and bond coat that affect strength of the critical interfaces and growth of the alumina scale and consequently the alumina scale adhesion. The factors will vary depending on the substrate/coating system. These factors include interdiffusion processes which change the chemistry of the coating and the substrate and the chemistry of the oxide scale. Changes related to these interdiffusion processes affect not only coating chemistry, but microstructure, creep resistance, fracture toughness, phase composition and other coating properties, as well as growth of the alumina scale.
Essentially, there is a tendency for aluminum (Al) from the aluminum-rich aluminide outer layer to migrate inward toward the substrate, while traditional alloying elements present in the superalloy, Co, Cr, W, Re, Ta, Mo, and Ti migrate from the substrate into the coating as a result of composition gradients between the underlying superalloy and the coating. Extensive interdiffusion occurs between the coating and the alloy as a result of high temperature exposure. Aluminum diffusion toward the substrate reduces the concentration of Al in the outer layer, thereby reducing the ability of the outer layer to regenerate the highly protective and adherent alumina scale. Simultaneously, the migration of Co, W, Re, Ta, Mo, and Ti likely degrades the protective properties of the alumina. Another result of diffusion of aluminum is the formation of a diffusion layer or zone into the airfoil wall which essentially means undesirable consumption of the airfoil wall.
One solution to the problem of growth of the diffusion layers on metal substrates used in the glass manufacturing industry is set forth in U.S. Pat. No. 5,756,223 ('223).
In order to prevent oxidation of substrate materials, a ceramic interlayer is interposed over the substrate. The ceramic interlayer then is overcoated with a layer of palladium or platinum or a combination of the two. The purpose of the interlayer is to prevent the oxidation of the substrate by inhibiting the migration of oxygen through the precious metal coating to the substrate. The ceramic interlayer acts as a getter for oxygen. While this is effective for the glass industry in prevention of oxidation of the substrate, it is not effective for a gas turbine surface, because substrate oxidation is not a problem, and a ceramic interlayer such as described in the '223 patent is not effective in prevention of coating deterioration in gas turbine service, the solution for the glass industry does not address the complex and extensive diffusion processes occurring between superalloys and their aluminum containing coatings.
What is needed is a diffusion barrier between the coating and the substrate alloy that prolongs coating life by extending the time the coating chemistry provides a protective and adherent alumina scale, while being essentially chemically compatible with the bond coat and the superalloy, thermodynamically or kinetically stable and highly adherent to both the substrate alloy and the bond coat. In addition, the diffusion barrier should have low solubility and interdiffusivity for Al and elements from the substrate, minimal coefficient of thermal expansion (CTE) mismatch with the underlying substrate and the overlying protective coating, high stability at service temperatures, and ease of deposition preferably using currently available application techniques such as plasma spray, physical vapor deposition processes such as sputtering or other such methods. Oxide ceramics in which the diffusion rate of aluminum is low are likely candidates for diffusion barriers. These ceramic materials typically exhibit adherence problems. Thickness of such diffusion barriers typically are limited to a few microns, which may not be sufficient to act as effective diffusion barriers for a prolonged time at elevated temperature exposure.